What are microwave detection systems (used in prisons)?

Your question peaked my interest so I found this:

The complete microwave sensor system consists of a

transmitter, receiver and mounting hardware. The transmitter and

receiver are pole-mounted and installed facing each other to form a volumetric zone of detection. Microwave protection systems can be used by themselves to provide intruder detection around an entire perimeter or as a gap-fill solution where another security system acts as the main perimeter sensor.

Thanks for a quick response. Nice summary because I needed something that wasn't an entire article! :)